Very early in the evolution of the internal combustion engine, it was discovered that the relatively high levels of noise emitted during operation of the engine could be controlled, to a large extent, by resonant sound muffling devices. At least as early as about a century ago, it was discovered that a major portion of the sound emitted by an internal combustion engine exited through the tail pipe, which serves the primary purpose of exhausting spent combustion gases.
The approach toward the attenuation of these undesirably high levels of noise was to pass air exiting an engine through an acoustic filter. In principle, either high pass acoustic filters or low pass acoustic filters may be employed to muffle sounds in a duct. For example, a low pass filter is useful in order to prevent the transmission of relatively high frequency sounds. On the other hand, the low frequencies of acoustic energy which are predominant in explosive discharges, such as those created by the explosion of a gun or found in an automobile exhaust system may be filtered out using a high pass filter.
Likewise, a combination of both high pass and low pass acoustic filters may be used to achieve the elimination of noise. The elimination of noise may be viewed as generally involving the cancellation of the alternating flow of gases, representing sound transmission, while not impeding the steady flow of gas out from the exhaust system which is necessary in order to discharge spent combustion products.
As a general rule, mufflers have volumes in the range of six to eight times the piston displacement of the engine and may contain baffles with or without holes. A primary aspect of their operation involves the cancellation of sound waves by interference, usually involving breaking the waves into two parts which follow different paths and meet again out of phase before leaving the muffler. Another important aspect is that exhaust back pressure must be minimized in any muffler design, insofar as an increase of only one psi in back pressure decreases the maximum power output of an engine by about 2.5%. About 1% of this loss is due to additional work being expended by the engine to exhaust the gases. The balance of the loss is due to the effects of increased gas pressure on volumetric efficiency.
Turning to the case of ventilating ducts, a degree of noise suppression is usually obtained by lining the ducts on at least two non-opposite walls with an efficient sound-absorbing material for a distance of three to six meters from both the inlet and the outlet. Where, due to the length of available duct, this is insufficient, additional noise suppression may be provided by introducing baffles into the duct and covering the baffles with sound-absorbing materials.
In the case of duct associated noise control systems, increased speed of air flow introduces additional noise through the generation of turbulence. This must be addressed by additional baffles and/or sound absorbing materials.
Some understanding of baffle filter systems may be obtained if we consider a quarter wavelength resonant cavity. Such a cavity, known as a Helmholz cavity is a chamber closed at one end and open at the other. Because it is a quarter wavelength in length, sounds entering the open end of the chamber pass through the chamber and are reflected back to the open end of the chamber with a phase delay of one-half a wavelength. The half wavelength delay is caused because the time of transit of the acoustic disturbance through the chamber includes a forward transmission path of one-quarter wavelength and a reflected transmission back to the open end of an additional quarter wavelength.
The result is a half wavelength or 180.degree. phase shift in the output of the cavity with respect to the sound passing over the top of the cavity. Because the signals are phase shifted with respect to each other by 180 degrees, and because, for a first approximation, we can assume that during the emission of a particular sound, the amplitude and frequency of one wavelength of the sound is substantially identical to the amplitude and frequency of the next wavelength produced by the source. Thus, a given undulation corresponding to one wavelength is exactly cancelled by the prior undulation of the sound which one wishes to cancel. Naturally, this is only true for sound having the particular frequency which results in a quarter wavelength relationship between the Helmholz cavity and the sound. However, if the frequency is not far removed from the resonant frequency of cancellation, the cancellation effect will still occur to a substantial extent.
In early automobile mufflers, the approach taken was to pass the exhaust gases over a matrix of baffles which together defined a plurality of tuned cavities. This structure acted as a filter and to a limited extent cancelled a range of sound frequencies produced by the internal combustion engine, propagated through the manifold to the tailpipe, and which would otherwise exit the engine in the form of acoustic disturbances.
Today, the quieting of such muffler systems is on the order of eight decibels.
Notwithstanding the numerous disadvantages of this sort of noise muffling system, modern mufflers remain substantially identical in their essentials. Generally, such prior art mufflers are constructed of sheet metal. More particularly, such mufflers comprise an outer shell or casing made of sheet metal and a sheet metal baffle structure secured within the casing. A path for the conduction of combustion gases and attendant acoustic disturbances is provided in the muffler adjacent the various noise absorbing cavities.
Because the exhaust gases are both hot and corrosive (being the product of the combustion of gasoline), they cause relatively quick corrosion and otherwise deteriorate the sheet metal components of the muffler. The result is that the muffler must be periodically replaced.
Still another problem with conventional mufflers is the viscous resistance which they provide to spent combustion products. Nor is the viscous resistance of the muffler of no significant effect. Rather, the resistance encountered by escaping combustion products is significant enough to adversely affect fuel efficiency and the concentration of pollutants in the exhaust gases. This is caused, in part, by the failure of the engine to exhaust spent combustion products from the cylinders with the same degree of efficiency that an internal combustion engine without a muffler achieves.
While, to some extent, the problems, involved in the rapid deterioration of automobile mufflers can be addressed through the use of relatively expensive alloys, such as certain types of stainless steel, and the use of relatively thick material, the additional cost of such high quality materials renders this uneconomical. Moreover, the additional labor costs involved in manufacturing mufflers with relatively thick sheet metal components adds cost which clearly makes such mufflers impractical.
Likewise, while it is conceivable that a muffler design including relatively wide passages for the exhaust of combustion products and numerous cavities to cancel sounds passing over them could improve the incomplete scavenging of spent gases from the cylinders, the increase in size of a device made using such an approach would make it impractical in the environment of today's automobile. Here, space is at a premium and even the present day relatively small muffler represents a significant portion of the volume of the automobile. In any event, the muffler is also often the lowest point on the automobile and thus represents the limitation on clearance over the road. In connection with this, it is noted that even in the case of diesel-engine trucks, where the problem of back pressure has required the use of relatively large mufflers and the aesthetics and size of the truck have allowed the use of large mufflers, adequate muffling of combustion noise has not been satisfactorily achieved by existing muffler systems.